From a medical perspective, the Anopheles gambiae complex is arguably the most important group of insects; it is the primary vector of malaria in Africa, and hence the world. The goal of these projects is to understand the evolutionary genetics of the group; this includes understanding the genetic basis of sterility barriers between species and the dynamics of gene exchange between species. Previous research has indicated that two closely related species may be exchanging genetic material by hybridizing in nature, the phenomenon known as introgression. But it appears that not all parts of a species genome are equally able to cross species boundaries. Using genetic markers (microsatellites) we intend to directly test which parts of the genome are responsible for sterility barriers. This will be done by making interspecific crosses, backcrossing fertile hybrid females to males of each species, assessing the reproductive status of the backcross progeny, and genotyping them for at least 24 microsatellites spread evenly across the genome. When regions responsible for sterility are identified, finer scale mapping will be done by increasing the density of microsatellite markers. In a related study, we will determine which parts of the genome are able to persist in freely breeding backcross populations for up to twenty generations. The second project concerns the dynamics of introgression. In particular, the question of whether the introgression is an ancient phenomenon or a recent, ongoing process will be addressed. This project will involve examination of DNA sequences in a chromosome inversion, the "a" inversion of the left arm of the second chromosome (2La), for which there is good evidence of interspecies transfer. The history of this inversion will be determined using phylogenetic procedures and statistical analysis for the number on introgression events most consistent with the data. Understanding whether the two most important vectors of malaria in the world (An. gambiae and An. arabiensis) are exchanging genes is vitally important to control measures aimed at these species.